Regulation of hydrogenase biosynthesis by nickel in Bradyrhizobium japonicum

Abstract

The addition of 5 μM nickel to heterotropically growing hydrogen uptake constitutive mutants resulted in up to a 10-fold increase in hydrogen uptake activity. Strain SR wild type cells required nickel for the derepression of hydrogenase in media treated to remove contaminating nickel and produced increasing levels of hydrogenase activity as a function of increasing nickel concentration. Immunoblots of SR wild type cultures derepressed with various concentrations of nickel revealed that the amount of anti-hydrogenase cross-reactive material synthesized is dependent on the amount of nickel in the media. Studies using chloramphenicol demonstrated that protein synthesis, in addition to nickel, was required for the synthesis of hydrogenase; inactive (nickel-free) hydrogenase protein does not accumulate during derepression in the absence of nickel. Thus the acquisition of hydrogenase activity by adding nickel during the course of derepression is not the result of nickel being inserted into pre-formed hydrogenase. Rifampicin inhibition studies suggested that nickel is required for the transcription of hydrogenase related genes. It is proposed that nickel, in addition to its structural role in the enzyme, plays a role in the regulation of hydrogenase expression in Bradyrhizobium japonicum.